1. Field of the Invention
The present invention relates to a waste solution discharging apparatus for waterless lithographic printing plates which is used to discharge waste solution including scrapings which are generated after developing.
2. Description of the Related Art
Various waterless PS plates in which a silicone layer is used as an ink-resistant layer have been proposed. A waterless PS plate in which a light-soluble or a light-insoluble photosensitive layer and a silicone rubber layer are laminated on a base is disclosed in Japanese Patent Application Publication Nos. 44-23042 and 46-16044. Japanese Patent Application Laid-Open Nos. 48-94504 and 50-50102 disclose a waterless PS plate in which a light-adhesive photosensitive layer and a silicone rubber layer are laminated on a base. With these plates, several tens of thousands of prints can be printed without the use of dampening water.
The above-mentioned waterless PS plates are formed by laminating a photosensitive layer and a silicone rubber layer on a supporting body made of aluminum or the like. The top surfaces of these layers are laminated with a protective film so that the surfaces are protected. When an image is printed onto the waterless PS plate, the photosensitive layer hardens in accordance with the amount of exposure, and further adheres to the silicone rubber layer. In this state, when developer is supplied to the surface, the photosensitive layer swells due to the developer as much as unexposed portions and portions where there is little amount of exposure, and the silicone rubber layer is peeled off. These scrapings of the peeled off silicone rubber layer are recovered in the developer recovery tank along with the excess developer. The recovered developing solution is returned by the circulating pump to a developing solution supplying portion, and is supplied to the waterless lithographic printing plate.
There are various developers used in developing processing, including the following examples:
______________________________________ Developer 1 diethylene glycol-mono-n-hexyl ether 15 part by weight diethylene glycol-mono-ethyl ether 17 part by weight pure water 68 part by weight Developer 2 diethylene glycol-mono-hexyl ether 10 part by weight Newcol B4SN (60% aqueous solution) 4.5 weight % pure water 85.5 part by weight ______________________________________
For waterless development, use of a processing solution containing no organic matter (hereinafter called developing solution) has been proposed (see Japanese Patent Application No. 3-9558, Publication No. 1). This improves the quality of the PS plate itself by making processing without use of organic matter possible. As a result, tap water can be used, and there is no need to dispose of organic matter. There is therefore no environmental pollution.
However, when water being used as developing solution is used repeatedly in order to conserve resources, the scrapings in the developing solution, which are mainly silicone rubber, are recovered to the developing solution recovery tank. The scrapings adhere to the interior of the developing solution resupplying tank, thereby dirtying the developing solution resupplying tank. As a result, it is necessary to periodically clean the developing solution resupplying tank, and the maintenance thereof is complex.
Efforts have been made to place a filter in the circulating path, which supplies the developing solution from the developing solution recovery tank to the developing tank, in order to remove the scrapings from the developing solution circulating in the developing tank, the developing recovery tank and the circulating path. However, in this case, the filter gradually becomes blocked so that the predetermined amount of developer cannot circulate. As a result, the filter must be periodically changed, which requires labor.
In order to solve the above drawback, the following method has been proposed. The surface portion of the developing solution within the developing solution resupplying tank, in which the scrapings are floating, is recovered to a separate tank (collection tank) so that the developing solution remaining in the developing solution recovery tank (the developing solution other than the surface portion) is separated. The separated surface portion of the developing solution, which includes the scrapings, flows into a conduit of a discharge pipe, whose opening is in a vicinity of the surface of the solution, and is discharged as so-called waste solution. The remaining developing solution is circulated and used. Discharge of only the waste solution containing many scrapings is thereby effected.
Further, the discharge pipe is formed from a vinyl chloride pipe such that the cost is low and the manufacturing is easy.
However, scrapings which should flow into the discharge pipe may remain due to contact resistance with the discharge pipe such that the scrapings may not flow smoothly into the discharge pipe.
FIG. 6 illustrates the relationship between the processed amount of the waterless PS plate and the residual rate of the scrapings (silicone scrapings). Even if a vinyl chloride pipe is used as the discharge pipe to discharge the scrapings from the developing solution recovery tank, the residual rate is 5% for a 10 m.sup.2 amount to be processed, 40% for a 20 m.sup.2 amount, and 50% for a 30 m.sup.2 amount. The residual rate of the scrapings rises steeply in accordance with an increase in the processed amount.
A cause of this phenomenon is that the wettability of the material of the discharge pipe greatly effects the residual rate. Wettability is exhibited in the surface tension of a solid body. Namely, as mentioned above, vinyl chloride is used as a material for conventional discharge pipes. The surface tension of vinyl chloride is 41.9 mN/m(dyn/cm). As the value of the surface tension increases, the contact resistance of the floating scrapings to the periphery of the discharge pipe increases, and it is easier for the scrapings to remain thereat.
Further, another cause is that the opening end portion of a conventional discharge pipe is cut in a direction perpendicular to the axis. Therefore, the opening surface faces straight up. Namely, because the scrapings have certain thicknesses, the undersides thereof encounter resistance when the scrapings cross over the angled portion of the outermost periphery of the opening portion end surface. This increases the likelihood of the scrapings remaining thereat.